Cysteine is used for the purpose of enhancing the flavor of foods and the like. Known production methods of cysteine include a protein decomposition method and a semi-synthetic method. The methods that are currently used in the main are the proteolysys method and the semisynthetic method. Although natural food materials having high cysteine contents have been demanded for the purpose of using them to enhance the flavor of foods, such natural food materials have little been known. On the other hand, it has been reported that heat- or enzyme-treatment of yeast extracts containing γ-glutamylcysteine may give rise to food materials having high cysteine contents (WO 00/30474).
γ-glutamylcysteine is synthesized from cysteine and glutamic acid as substrates by the function of γ-glutamylcysteine synthetase. On the other hand, glutathione is synthesized from γ-glutamylcysteine and glycine as substrates by the function of glutathione synthetase. Therefore, as a method of breeding a yeast that accumulates γ-glutamylcysteine in high contents, there may be proposed disruption of a gene that encodes glutathione synthetase. Yeasts whose genes that encode glutathione synthetase have been disrupted are reported in WO 00/30474; Otake et al., Agri. Biol. Chem., 54(12), 3145-3150, 1990; Chris et al., Molecular Biology of the Cell., 8, 1699-1707, 1997; Inoue et al., Biochimica et Biophysica Acta, 1395(1998) 315-320.
However, each of the above-mentioned yeasts has a defect that their growth rates are decreased to a large extent. Further, Otake, et al. reported that the yeast whose gene encoding glutathione synthetase has been disrupted shows a bad growth under a culture in a medium containing no glutathione in comparison with a culture in a medium containing glutathione (Otake et al., Agri. Biol. Chem., 54(12), 3145-3150, 1990). However, since media containing glutathione in abundance are generally expensive and glutathione itself is also expensive, such media are not preferable for industrial use. On the other hand, it would be also inappropriate to culture the above-mentioned yeasts at high densities in inexpensive media containing insufficient amounts of glutathione for use on an industrial level.